The present invention relates to information transmission using light in an information processor. More particularly, the present invention relates to an optical link module capable of linking light beams emitted from light emitting devices with high efficiency in a compact volume, an optical interconnection method, an information processor using the optical link module, a signal transfer method in the information processor, a prism used in the optical link module and a method of manufacturing the prism.
With the recent advancement in information-processing technologies, the capacity and the speed of communication between and within housings of information processors have been dramatically advanced. In the immediate future, a speed of a communication request transmitted from one piece of processor board will exceed 1 Tbps. Accordingly, it is predicted that it will be impossible to load connectors for information communications on a card edge by use of a conventional signal communication method. In this case, a transfer rate before coding is predicted to be several Gbps per channel.
In such a case, a signal communication system using an optical link with a large information transfer rate and capacity is considered to be necessary for resolving the above-described problem to a great extent. However, conventional optical link modules are mainly for middle-/long-distance applications such as the Internet and a telephone system and thus the number of channels thereof is overwhelmingly smaller than that of an optical link module used for a computer. Consequently, the art has concentrated on link modules having only one fiber channel built-in by means of high-speed modulation of several 10 Gbps or more by use of a large device.
As to the fiber used for the above-described purpose, because of the necessity of suppressing mode dispersion noise to be low across a long distance and securing a high transmission rate, it is required to use a single mode fiber with a core diameter of about 5 micrometers. Moreover, as to a laser diode, because of the necessity of transmitting light across a long distance, a laser diode with a high output power is required and thus an edge emitting laser is used. Accordingly, the size of a light emitting part of the laser diode is 1 micrometer or less. In assembly thereof, alignment of an optical axis, referred to herein as “active alignment”, is always required and thus an assembly cost becomes high.
Meanwhile, for applications to information processors in computers such as a microcomputer, a server, a main frame, a supercomputer and a massively parallel processing computer, requests are completely different from one another. A transmission distance required in the inside of the information processor is at a minimum on the order of several decimeters to several meters, which is very different from the case of a long-distance communication. As the optical fiber, a low-cost multimode fiber sufficiently guarantees a high-speed transmission band. However, in order to realize high-density packaging on a circuit board, individual components are required to be extremely compact. Moreover, it is also assumed that signals between integrated circuits within the circuit board are transmitted directly between circuit boards. Thus, smallness of a time delay, that is, latency is demanded more than throughput per channel. Meanwhile, a required modulation speed is assumed to be about 10 Gbps within a range capable of driving a CMOS logic IC as a semiconductor element. In addition, considering arrangement of many circuits, it is required to maintain the cost per channel to be significantly low.
Furthermore, in recent years, a laser diode (LD) called a vertical cavity surface emission laser (VCSEL), which emits light vertically to a surface of a wafer substrate, has been developed and available in the market. Thus, two-dimensional parallel integration has become possible. Meanwhile, a light receiving part includes a photodetector (hereinafter referred to as a PD) which is approximately formed of photoelectric convertors such as photodiodes and phototransistors. Since the PD receives light on its surface, there has been conventionally proposed a technology capable of two-dimensional integration. To be more specific, the VCSEL includes dot-like light-emitting devices having a diameter of about 5 to 10 micrometers, which are arranged two-dimensionally, and can directly perform on/off modulation up to about 10 Gbps or more. Moreover, the VCSEL has an advantage that the VCSEL can be manufactured at a lower cost and in larger quantities than an edge emission laser diode in which a light output edge is formed by use of crystal cleavage as in the conventional technology. Furthermore, it is known that the VCSEL can obtain a sufficient light coupling efficiency just by directly facing a graded-index multimode fiber with a core diameter of 50 micrometers and can realize a low-cost connector.
Moreover, a ribbon-shaped fiber bundle is also known, which can transfer 10 Gbps up to 100 meters without lowering a band characteristic and has the graded-index multimode fibers with a core diameter of 50 micrometers, the fibers being mass-produced and bundled in parallel. There has been advancement in standardization of a two-dimensional fiber array connector for mutually connecting fiber bundles in which several pieces of the ribbon-shaped fiber bundles are further bundled together to increase a degree of parallelization.
Considering a request for miniaturizing in the case of using the fiber bundle as a wiring of an information processor, it is preferable to adopt a structure in which the optical fiber is attached/detached by mounting an optical link module on a card edge. To adopt the above constitution, a direction of attaching/detaching the optical fiber bundle is required that is even with the circuit board surface. Specifically, it is preferable that ends of the two-dimensionally arranged optical fibers are vertical with respect to the circuit board surface. By way of contrast, an IC in which LDs and PDs are arranged two-dimensionally is mounted in parallel with (even with) a board surface. Thus, in order for the optical fiber to be coupled with the LD/PD, an optical bend is required which bends light at a right angle.
Moreover, the optical fiber is bent by utilizing the flexibility of the optical fiber while setting the direction of attaching/detaching the optical fiber bundle to be vertical to the circuit board. Thus, the LD and the PD can be directly connected to each other. However, more than 30 centimeters curvature radius is usually required to bend the optical fiber. Thus, racks for arranging a number of circuit boards are bulky and it is difficult to meet requests, such as for the miniaturization of an information processor and the saving of space therein. Furthermore, a link method using the flexibility of the above-described optical fiber has an inconvenience that it is impossible to achieve speeding-up by shortening a signal transmission distance by high-density packaging, which is an original aim.
An optical link module including parallel optical fiber bundles for the above-described information processor is described in Japanese Patent Laid-Open No. 2001-242358, with a one-dimensional array electrical turn link module. In a one-dimensional array optical link module described in Japanese Patent Laid-Open No. 2001-242358, optical fibers are disposed in parallel for 12 channels to form one array. In Japanese Patent Laid-Open No. 2001-242358, there is disclosed an optical link module, in which a VCSEL array IC and a PD array IC are vertically disposed and are connected to horizontally disposed driver IC and amplifier IC by use of flexible wiring or connected to an end of a thick copper wiring pattern by use of wire bonding.
When there are one or two parallel arrays, with an interval of 250 micrometers per channel, an electrical wiring pattern can be formed in a flexible cable. However, the optical link module disclosed in Japanese Patent Laid-Open No. 2001-242358 has a problem that an electrically bendable wiring pattern cannot be formed. Furthermore, a method of connecting the LD/PD to four sides of the IC in a space by bending the flexible cable so as to surround the space has assembly problems and poor high-frequency properties.
Furthermore, there is also known a technology of deflecting light beams for one channel by use of a prism or a mirror. For example, in Japanese Patent Laid-Open Nos. 2000-321453, 2001-141966, Hei 9 (1997)-307134 and Hei 7 (1995)-202350, there is disclosed a method of guiding light beams to an optical fiber end by use of an optical waveguide without specially placing a prism by utilizing the fact that the same effect as the total reflection of a prism is achieved by cutting the optical fiber end at a 45-degree angle with respect to a transmission direction of the light beams.
Moreover, in Japanese Patent Laid-Open No. Hei 8 (1996)-29161, there is disclosed a prism with a lens, in which positioning in assembly is simplified. However, the optical bend disclosed therein is a one-dimensional fiber array, in which an optical path difference between channels is constant and a distance from a light exit end to a light incident end can be shortened. However, when a two-dimensional fiber array and two-dimensional light-emitting/light-receiving devices are optically coupled together, two faces of a triangle prism, which are orthogonal to each other, are faced to a two-dimensional laser diode array. Thus, the optical element is required to have the same size as the array. This means that, when there are n of two-dimensional arrays, a distance through which light travels is increased by n times compared to the case of the one-dimensional array. Accordingly, if the conventional technology is applied as it is, there is a difficulty that the optical element cannot be realized within a range which satisfies imaging conditions of a thick lens.
Japanese Patent Laid-Open No. Hei 7 (1995)-261060, describes “optical packaging” in which two pairs of two-dimensional fiber arrays are coupled together by combining a prism and a hologram optical system. However, in Japanese Patent Laid-Open No. Hei 7 (1995)-261060, the optical packaging which achieves the above-described function is disclosed merely on a conceptual basis and the kind of a hologram to be used is not described. In addition, values of a coupling efficiency and the like are not evaluated therein. Moreover, in the optical packaging described in Japanese Patent Laid-Open No. Hei 7 (1995)-261060, evaluation of the coupling efficiency by simulations and the like is not disclosed and nothing specific about effects obtained in actual applications is described.
Even if an attempt is made to solve the above-described problem only by use of a prism without using a lens array, this is not altogether satisfactory. Specifically, when an optical bend or turn is formed by use of a prism (or a mirror) as it is for a two-dimensional array, there is crosstalk in an adjacent channel as described later and the power of the light beam is greatly lost. A main reason for this problem is that light emitted from an LD spreads at least at 8 degrees or more on one side by diffraction at an opening portion of a VCSEL and thus a light beam has a spread of 10 to 20 degrees on one side. Furthermore, light emitted from a fiber is a multimode and thus spreads at an angle determined by NA of the fiber (since nominal NA<=>0.2 in a 50-micrometer core, 12 degrees at one side and strength of about 10%). If this light beam has to travel across a distance for 5 arrays of 12 arrays times 5 rows, a luminous flux spreads out to an adjacent channel. Thus, it is not possible to apply the optical element as it is to the signal wiring element in the information processor.
FIG. 14 shows, regarding the above-described inconvenience, an illustrative reflection behavior of a luminous flux in the case of using a prism only for both of an incidence plane and a reflection plane. FIG. 14 shows ray tracing simulation of a case where fiber ends of 12 arrays by 5 rows are coupled with LD/PD by use of a simple rectangular prism. FIG. 14 schematically shows ray tracing for one transmission channel (TCH) and for one reception channel (RCH). The ray tracing is performed assuming that ends of five optical fibers OF with a bore diameter of 50 micrometers are arranged by a pitch of 250 micrometers at a bottom of a prism 200.
Meanwhile, in the reception channel RCH, light is emitted upward from the optical fiber OF, a large part of the light undergoes total reflection at a 45-degree plane and the reflected light enters photodiodes (PD) with a bore diameter of 40 micrometers, which are disposed to be approximately adjacent to each other on the left side of the prism. Note that, in FIG. 14, the light beam entering its target PD is stopped at that point.
As shown in FIG. 14, a large part of light escapes from the target PD and is made incident on the adjacent PD above the target PD by 250 micrometers. Moreover, in the transmission channel TCH, light emitted rightward from an LD with a bore diameter of 6 micrometers, which is placed at the left end of the prism while nearly touching the prism, spreads and is reflected on the 45-degree plane before entering a target optical fiber OF, which is second from the right bottom. Similarly to the reception channel RCH, in the transmission channel TCH, it is obvious that a large part of light escapes from the target to form stray light and is made incident on the adjacent optical fiber on the right of the target by 250 micrometers.
Even if the pitches of the arranged optical fibers and PDs are increased to grow in size in order to avoid such a crosstalk, the distance through which the luminous flux has to travel is accordingly increased and there arises an inconvenience of a lowered coupling efficiency. Moreover, when the fiber end, the PD and the LD are separated from the prism, a distance through which light travels in the air (the air has a refractive index of 1, which is lower than that of the prism) is increased and thus there arises an inconvenience that characteristics are more deteriorated.
Clearly, a need exists for an optical link module for a two-dimensional parallel fiber array that is capable of obviating the above-described inconveniences, an optical interconnection method using the optical link module and an information processor including the optical link module have been heretofore demanded.